Coralbells (Heuchera) Hardiness Research 2009-2010

Dr. Leonard Perry, University of Vermont

The last decade or two has seen an explosion of coralbells (Heuchera) on the market, with little known about their hardiness, particularly under extreme cold climates. Zone ratings are often contradictory, cover a wide range, and in many cases are conservative, similar to many other new perennial introductions. One group of coralbells—those with villosa or hairy alumroot parentage—have been among the more popular of this genus. Growers have raised questions as to their hardiness, the plants having been bred in France and the west coast, and with parents from different regions. Native to the southeastern U.S., the species is often listed as growing in USDA zones 6 to 9. So the purposes of this study were, 1. To field test many new coralbell introductions in zone 4a for at least two winters, and 2. To perform controlled freezing studies on potted and overwintered coralbells of villosa heritage.

Controlled Freezing Studies

Five cultivars were obtained in summer 2009, potted into 4-inch square pots, and grown through fall outdoors in northern Vermont under 40 percent shade. Plants were then moved in early November into a double-glazed rigid poly house with 75 percent shade (from a heat curtain inside and white poly covering outside), where soil temperatures were maintained at an average 40°F, with a range of 36° to 44°F. Occasionally temperatures would go higher for a few hours on sunny days, reaching 54°F, or for a few hours during cold nights to 28 to 30°F for a few hours in early morning.

Plants were placed into controlled temperature freezers on two dates, December 11, 2009 and February 6, 2010. On each date, plants were allowed to drop to 28°F and become uniformly frozen over 2 days. Then they were dropped to each target temperature (23°F, 18°F, 13°F, and 9°F) over a roughly two hour period, and allowed to remain at those temperatures for a half hour, before removing one set of plants and proceeding to the next temperature. This procedure is the same as has been used for previous such hardiness studies we’ve conducted over the past 20 years. Plants were then removed and allowed to return to greenhouse temperatures, and grown on. They were rated and moved outside in early April. Ratings were based on relative percent growth, both previous and new. There were 6 replicate plants of each cultivar, for each temperature. Within each cultivar, data was analyzed statistically using Analysis of Variance, with means separated if differences existed by Tukey’s procedure.

Each cultivar was frozen in the same freezer to eliminate variability between freezers. Two cultivars were in one freezer, three in the other. On the second freezing date in February, one freezer had an unexplained and abnormal sudden drop in temperature after initial freezing. Temperatures dropped below 18° and 13°F, so data were not available for these temperatures for Frosted Violet, Mocha, and Tiramisu. The quicker than normal drop also may have affected the 23°F results. Instead, temperatures were held below 13°F for either 18 or 24 hours. No plants survived either of these extreme treatments. These were extreme, as plants would seldom if ever experience such extremes in the field or in pots. In the field, over almost two decades of winter soil measurements in USDA zone 4a, temperatures haven’t dropped below 28°F, and seldom go below 30°F. Past studies, and discussions with growers, indicate that even in cold climates, pot temperatures in unheated overwintering houses, when pots are on the ground, don’t drop below 21°F to 23°F during extreme events. Of course this may vary with snow cover outside, and type of greenhouse coverings.

Coralbells (Heuchera villosa) ratings after Controlled Freezing

1=dead, 3=saleable, 5=excellent regrowth

Statistics: NS=no significant differences, p=significance level, MSE=Mean Square error measure of variability; where significance, those numbers within a row, followed by the same letter, are not significantly different.

Cultivar	date	-2°C/ 28°F	-5°C/ 23°F	-8°C/ 18°F	-11°C/ 13°F	-14°C/ 9°F	statistics
Blackout	Dec.	5.0	4.8	4.3	4.3	3.5	NS, p=0.07, MSE=0.3
	Feb.	4.3ab	5.0a	3.8ab	3.2bc	1.7c	P<0.001, MSE=0.3
Caramel	Dec.	5.0a	4.2ab	2.8bc	3.5abc	2.2c	P<0.001, MSE=0.5
	Feb.	4.5a	4.7a	2.7b	1.0c	1.5b	P<0.001, MSE=0.3
Frosted Violet	Dec.	4.8a	4.7a	4.7a	3.2ab	2.0b	P<0.001, MSE=0.2
	Feb.	4.5a	2.0b	--	--	1.0b	P<0.001, MSE=0.2
Mocha	Dec.	4.8a	4.7a	4.8a	3.3b	2.7b	P<0.001, MSE=0.2
	Feb.	4.5a	3.8a	--	--	1.3b	P<0.001, MSE=0.3
Tiramisu	Dec.	3.5	3.0	3.5	3.5	2.0	NS, P=0.04, MSE=0.4
	Feb.	4.7a	3.2b	--	--	1.0c	P<0.001, MSE=0.1

Blackout showed no differences among treatments in the December freezing, surviving all the way to 9°F. After the February freezing, there were significant differences, with plants rating saleable (rating 3 or above) to 13°F. The top rated treatments for both dates were 18°F and above.
Caramel had significant regrowth differences after both freezings. In December, plants remained saleable after freezing to 13°F, yet only down to 18°F in February. The top rated treatments for both dates were 23°F and above.
Frosted Violet was similar to Caramel in December, saleable after freezing to 13°F. In February, it was only saleable to 28°F, likely due to reasons mentioned above, and with no treatments to 18°F or 13°F. The top rated treatments for the December freezing were 13°F and above.
Mocha actually was saleable after freezing all the way to 9°F in December, similar to Blackout, even though there were some treatment differences. Similar to Frosted Violet, a couple treatments weren’t done in February, yet this cultivar was saleable after freezing to at least 23°F (3.8 rating). Although the rating at 23°F in February was lower than that in December at this temperature (4.7), neither was significantly different from the best treatment on each date. The top rated treatments for the December freezing were 18°F and above.
Tiramisu had lower ratings in December than all other cultivars, only being marginally saleable after freezing, but being so after all treatments. None were really “top rated” (all below 4.0), while in February the top rated treatment was only 28°F (4/7). Even though 9°F wasn’t rated as saleable (2.0), statistically it was not significantly different from the other treatments. In February, as with the previous two cultivars, a couple treatments weren’t done. It was saleable, similar to Mocha, after freezing to at least 23°F. The ratings from the 23°F treatment on both months were similar even though the one in February (3.2) was significantly lower than the best treatment (4.7). The highest freezing temperature in February resulted in a higher rating (4.7) than from this same 28°F in December (3.5).

Several overall observations from these studies were:

Plants seemed hardened by December, surviving, if not saleable, to the lowest temperature (8°F).
All 5 cultivars had saleable regrowth in spring after freezing to 13°F in December.
Blackout had saleable regrowth in spring after freezing to 13°F in February too, while Caramel was saleable only to 18°F.
Tiramisu had saleable but less vigorous, and lower rated, regrowth than the other cultivars after all treatments.
Comparing top rated treatments on both dates, Blackout appeared slightly more hardy (18°F and above) than Caramel (23°F and above). Comparing top rated treatments for the other cultivars from just the complete data set from December, Frosted Violet and Tiramisu appeared the most hardy (13°F and above), followed by Mocha (18°F and above). However, for all treatments except the lowest (13°F), Tiramisu had lower ratings than the other two cultivars, just marginally saleable.

It appears there may be a difference among freezing dates in winter, plants perhaps slightly more hardy and able to withstand low temperatures in mid-December than mid-February. Most were able to withstand a brief (2 hour) exposure to 23°F and still have good regrowth. When fully hardened, they were able to withstand much lower (13°F) and still be saleable. It would be interesting to see how long at each of these temperatures plants would recover similarly, only being at these target temperatures for a half hour in this study.

It was surprising, given questions raised by growers and from earlier field plantings, of this hardiness. The difference and performance of the best—Blackout, and the poorest—Tiramisu, was not surprising given earlier field trials. Although a couple were the most hardy of the cultivars after the first freezing, it would be interesting had there been data from the second freezing to see if they were as hardy then. Five different cultivars will be frozen similarly during winter 2010-2011, and the results compared to these initial five. All will then be compared to two years of field results.

Field Trials

Just over two dozen cultivars were planted in the field in a stony loam, in early fall 2008. Although listed in zone 4a, both winters barely got below -20°F so were in reality 4b. Water was given for establishment, and as needed during dry periods, and compost side-dressed yearly along with 5-3-4 in early summer, otherwise minimal care was given to similar a home situation and to maximize plant stress. This seems verified by the fact that three cultivars that didn’t survive the first year in the field trials did survive fine in landscape beds receiving more attention. Snow cover was adequate, with winter soil temperatures seldom dropping to, or much below, 30°F. Of these initial plantings, 12 died after the first winter, and 14 survived both winters (see ratings below), and one died after the second winter. Of those that died, as many as were available are being tried a second winter. The survivors will be continued, along with a second season for 40 additional cultivars, with more added. Of those surviving and rating highest, the most vigorous, with excellent and best flower display, was Silver Lode.

Field survival ratings of Coralbells after 2 winters (2008-2010) in USDA zone 4b in Vermont.

Ratings were 1=just living to 3=excellent, d=died after second winter.

Cultivar	rating
Amethyst Mist	1.5
Autumn Bride	2
Bronze Wave	2
Dolce Creme de Menthe	3
Dolce Licorice	3
Dolce Mocha Mint	2.5
Georgia Peach	1
Ginger Ale	1.5
Green Ivory	3
Mahogany	d
Mocha	2.5
Obsidian	3
Purple Petticoats	3
Silver Lode	3
Starry Night	2.5

Several other cultivars died after one year and are currently under retrial; 40 additional cultivars have only been tested for one year.

Appreciation is expressed to the New Hampshire Horticulture Endowment, and to North Creek Nurseries (Landenberg, PA), for their support and making this study possible.

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